1. Field of the Invention
This invention relates to a method for drilling generally horizontal boreholes in a subterranean earth formation, and more particularly to a method of adjusting the trajectory of such a borehole to maintain same within a seam of coal or the like.
The use of rotary drilling to form long generally horizontal gas relief holes in coal beds is known in the art as a means for degasifying a coal bed in advance of mining. These gas relief holes are either vented or connected to a vacuum source to remove methane from a coal bed. The greatest problem encountered in drilling these gas relief holes is that of maintaining the bit trajectory within the coal seam such that the resulting holes are actually through the coal seam rather than through an overlying or underlying formation.
2. Description of the Prior Art
The state of the art to which the present invention pertains is set forth in detail in a Bureau of Mines Report of Investigations published in 1975 numbered 8097 and entitled "Rotary Drilling Holes in Coalbeds for Degasification" by Cervik et al, available in the U.S. Department of the Interior Library. That report describes the use of rotary drill bits attached to drill rods and maintained in a desired trajectory by a combination of bit thrust, rotational speed and drill rod centralizer spacing. That report further notes that locating a centralizer or stabilizer near the drill bit will cause a slight upward trajectory with proper drill thrust and bit rotational speed, and further notes that a downward trajectory can be obtained by locating a centralizer several meters behind the bit. However, relocating a centralizer to cause a change in bit trajectory has previously involved removal of the drill string from the borehole. Such a procedure is time-consuming and unproductive.
A sliding stabilizer assembly for controlling bit trajectory without the necessity of removing the drill string is described in U.S. application Ser. No. 796,093, filed May 12, 1977, and now U.S. Pat. No. 4,108,256.
There has been a continuing need, prior to the present invention, for an improved method for controlling the trajectory of horizontal gas relief holes in coal seams.